The prior art discloses patents for valves of the packed spool, lap spool or proportional flow version and valves which employ cartridge units that may be assembled outside of the valve body and moved into the bore in the body of the unit, for example, U.S. Pat. No. 4,096,880 to Lemon et al and U.S. Pat. No. 4,220,174. Such prior art constructions do not provide a valve which can be used in connection with various activating means such as solenoids, levers, pedals and the like.
U.S. Pat. No. 4,877,057 to Christensen discloses a pressure equalizing valve with a spool disposed in a valve body to regulate fluid communication from an inlet port to outlet ports in response to a pressure differential between first and second port disposed on first and second end faces of the valve body. However, the valve does not provide a means for differentiating pressures.
Prior to the present invention cryogenic tanks were commonly equipped with a differential pressure gauge so that an operator could gauge the amount of liquid in the tank. One side of the gauge was connected to the top of the tank and the other side of the gauge was connected to the bottom. The amount of liquid in the tank was then determined by inference by the difference of gauge pressure. In the system, the gauge was protected by an equalizer valve which was connected to two lines which ran on both sides of the gauge.
In operation, the operator must open the equalizer valve to allow the gauge to come to an equilibrium on both sides of the gauge. After the equalizer is opened, the blocking valves may be closed. This will permit the gauge to be calibrated or removed from the system. To reverse the operation and place the gauge in service requires the blocking valves to be opened and then the equalizer valve to be closed. If this is not performed in proper sequence in either operation, an inadvertent high pressure may occur on one side of the gauge and inactivate the gauge.
It should be understood that the term "liquid" as used herein can also refer to a gas which is the result of the liquid being heated to ambient temperature as is the case with cryogenic fluids.